1. Field of the Invention
The present invention relates to a phase shift interferometer. Particularly, the present invention relates to a Fizeau-type phase shift interferometer.
2. Description of Related Art
As a measuring device for measuring irregularity of a surface-to-be-measured, there has been known a phase shift interferometer, and particularly there has been known a Fizeau-type interferometer as shown in FIG. 13 for measuring irregularity of a wide surface (refer to, for example, Japanese Patent Laid-Open Publication No. Hei 11-337321).
The optical path of the Fizeau-type phase shift interferometer as shown in FIG. 13 will be described below.
As shown in FIG. 13, the light (L10) emitted from a light source 11 is brought to a parallel beam by a collimator lens system 12, passed through (L11) a half mirror 13, and incident on the surface of the workpiece.
The light (L10) emitted from the light source 11 is a linearly polarized light.
Herein a quarter wavelength plate 14, which also serves as a reference surface, is provided between the half mirror 13 and the workpiece.
A portion of the light (L11) incident on the quarter wavelength plate 14 from the light source 11 is reflected by the quarter wavelength plate 14, and the rest passes through the quarter wavelength plate 14.
The light (L12) reflected by the quarter wavelength plate 14 returns to the half mirror 13 as a reference light.
The light (L13) passed through the quarter wavelength plate 14 is irradiated on the surface of the workpiece, reflected by the surface of the workpiece as an object light (L14) to return to the quarter wavelength plate 14, and then passed through the quarter wavelength plate 14 to be incident on the half mirror 13.
At this time, since the object light (L14) from the surface of the workpiece passes through the quarter wavelength plate two times, the vibration direction thereof is rotated by 90°, and therefore the polarizing direction thereof is orthogonal to that of the reference light (L12).
Thus, as non-interfering light beam, the reference light (L12) and the object light (L14) return to the half mirror 13 through a common optical path without interfering with each other.
The non-interfering light beam returned to the half mirror 13 is reflected (L16) by the half mirror 13, then reflected by the reflecting mirror 15, and then passed through (L17) the quarter wavelength plate 16.
By passing through the quarter wavelength plate 16, the object light (L14) and the reference light (L12) contained in the non-interfering light beam are converted into circularly polarized lights having rotating directions opposite to each other.
The light (L17) passed through the quarter wavelength plate 16 is split into three light beams (L18, L19, L20) by a first half mirror 17, a second half mirror 18, and a reflecting mirror 19 provided in the optical path.
Polarizers 20, 21, 22 and CCD cameras 23, 24, 25 are respectively provided in the optical paths of the split light beams (L18, L19, L20).
Herein the first polarizer 20 provided in the optical path of the first light beam (L18) reflected by the first half mirror 17, the second polarizer 21 provided in the optical path of the second light beam (L19) reflected by the second half mirror 18, and the third polarizer 22 provided in the optical path of the third light beam (L20) reflected by the third half mirror 19 respectively have transmission axis angles different from each other.
For example, the first polarizer 20 has a transmission axis angle of 0°, the second polarizer 21 has a transmission axis angle of 45°, and the third polarizer 22 has a transmission axis angle of 90°.
Thus three interference fringes having phases different from each other by 90° are imaged by the respective CCD cameras.
Further, the images of the interference fringes are input to a predetermined analyzing section, and phase information of the surface of the workpiece can be acquired by comparing image strengths in each point among the three interference fringes.
Thus the surface shape of the workpiece can be measured.
In such an arrangement, with the Fizeau-type phase shift interferometer, though the surface shape of the workpiece can be measured in a wide range by emitting a wide parallel light beam into the surface of the workpiece, there is a problem that the structure is actually impracticable.
With the above arrangement, in order to multiplex the object light and the reference light without interfering with each other, the quarter wavelength plate 14 is disposed in the optical path, and the object light passes through the quarter wavelength plate 14 two times so that the object light and the reference light become polarized lights orthogonal to each other. However, to produce a quarter wavelength plate 14 with a large opening is actually very difficult.
In other words, since a wavelength plate is a flat plate obtained by cutting a double refraction crystal in a predetermined direction, it is very difficult to obtain a good quality wavelength plate with a wide opening, and the manufacturing cost will be extremely high.
Thus it is desired to provide a practicable Fizeau-type phase shift interferometer.